After two years, the only scratch on my old HTC Desire HD's screen came from when it bounced off a flagstone floor. Having read some pretty damning reviews, I've decided - wisely or not - to forego insurance, but to invest in a decent case for my S3. So I've bought an Otterbox Commuter, which offers a lot of protection, including a high ridge around the screen. This should prevent any screen damage if I should drop the phone, unless the circumstances are extreme and I'm very unlucky. And, should that be the case, I seriously doubt that a screen protector would make any difference.
The Otterbox did come with a screen protector, but I'm hopeless at fitting them, and no one in the shop fancied doing it either. Anyway, I never keep the phone in the same pocket as anything that could scratch it, and am wise enough not to play with my phone in a sand-pit!
I'll tell you in two-year's time whether my decisions were wise!